More Secret Wars, Siskoid? Well, sort of. "The War Between Superman and Jimmy Olsen!" is more of a Convergence thing. That is to say, there's a domed city in the story, unnecessary fighting, and oh yeah, it kinda sucks. Convergence trifecta.
So it starts with Superman bringing Jimmy Olsen to the Fortress of Solitude to brag about his trophies. Note: Only one of these will be responsible for what happens next.
As they fly back to Metropolis, Jimmy whines about not being able to see anything, bundled as he is in Superman's protective cape. And as Clark Kent, Superman's none too pleased about Jimmy asking where the Fortress is for his inevitable article on Superman's braggery. He waves his incomplete notes in Clark's face, eliciting some frowns one might chalk up to professional jealousy. Just then, the "action" is interrupted by the cops who need to take Clark and Jimmy into protective custody until they can testify at the Double-X Gang's trial. When Jimmy hears they might have to stay at a safe house for up to three days, he rips the drunk tank's door off its hinges and escapes.
Superman's working theory is that Jimmy inhaled addictive super-strength spores, so he tells the cops he'll personally deliver Clark to court and flies after Jimmy. Jimmy's plan is to cause all sorts of trouble until Superman tells him where the Fortress is. He tries to derail a train, freezes Metropolis Bay over, but it's not until he breaks a Superman statue that the Man of Steel finally takes notice.
OH NO HE DIDN'T!!! The action moves to a military base where Jimmy tries to crash planes, pushes tanks into buildings and blows up the munitions dump is what has to be the most spectacular cry for attention in Jimmy Olsen history.
And that's saying something. But see how Superman lets the cat out of the bag re: a kryptonite boulder out at sea? Well, Jimmy doesn't take long to show he IS ready to go "all the way". (Oh, double-entendres...)
With the tonged boulder as leverage, Jimmy makes Superman bring him to the Fortress, with the freckled traitor drops the thing right on Superman before plunging into the mountain. That's when it's revealed that he's not Jimmy Olsen at all, but the Kandorian criminal scientist El Gar Kur who built a body switching machine to escape the bottled city.
And amazingly, he looks JUST LIKE JIMMY. Just like the Kandorian who takes the mantle of Flamebird later. Just like there are a couple of Superman lookalikes in the city. What is wrong with the Kryptonian genome, anyway? And hey, why did EGK wait for Jimmy to get a tour before replacing someone? Wouldn't it have been a lot safer to switch places with Superman himself? Or does the machine only work on your perfect twin from the normal sized world? Then again, EGK isn't the smartest tool in the Kandorian shed.
All this time, he's wanted to return to the Fortress so he could smash the city to bits, ensuring no one would ever tell Superman who he really was. Like this act wouldn't create a lot of tiny super-powered Kandorians putting a stop to his plans. But it never comes to that because this is the Silver Age and...
Ok, so to recap: Superman switched out for a Superman robot at some point, and was actually hiding inside the fake kryptonite boulder which he'd suggested "Jimmy" use to defeat him. And apparently, he's known this was an impostor since "Clark" saw Jimmy's notes written in someone else's handwriting. And to keep his suspicions secret EVEN FROM THE READERS, Superman then proceeded to fill his thought bubbles with red herrings, wondering why Jimmy was acting out of character, and so on. I CALL SHENANIGANS! I need for this story to be over. Do your thing, Kandor.
At least Superman calls EGK out on his terrible use of super-powers. That nagged me all the way through. So easy to find the Fortress with that suite of powers, geez. And so Jimmy is switched back into proper place, and hahaha, he cut himself on the boulder thing, so it's really him, hahaha, mortals, haha.